This invention relates to azetidinones which are useful in the preparation of 3-hydroxy cephalosporin derivatives. Several substituted azetidinones are currently known. An important utility for azetidinones is in the preparation of cephalosporin compounds which serve as intermediates for preparing cephalosporin antibacterial agents. For example, Nayler et al. converted benzyl 6.beta.-(triphenylmethylamino)penicillinate into a 4-(3-phenylprop-2-ynylthio) azetidinone, which was subsequently converted to a 3-substituted methyl cephalosporin, as described in detail in J. Chem. Soc. C 58 (1973). Heusler et al., in U.S. Pat. No. 3,842,072, described certain 3-amido-4-(1,1-dimethyl-2-hydroxyethylthio)-azetidin-2-ones, prepared by reducing 3-hydroxy penams with a hydride reducing agent. Additionally, Kukolja et al. described a group of 3-chlorosulfinyl- 3-imido-azetidin-4-ones which are useful for preparing desacetoxy cephalosporins; see U.S. Pat. No. 3,843,682.
Azetidin-2-ones substituted with 3-bromopropenyl and formylthio groups are hitherto unknown. The azetidinones currently available are not intermediates in the preparation of 3-hydroxy-3-cephems, an important class of cephalosporin compounds. An object of this invention is therefore to provide novel azetidin-2-ones, which compounds are especially useful for preparing 3-hydroxy-3-cephem compounds. An additional object of this invention is to provide a process for preparing 1-(1-protected carboxy-2-hydroxy-3-bromo-1-propenyl)-3-amido(or imido)-4-formylthio-azetidin-2-ones.